Crazy questions with not-so-crazy answers

 kw: book reviews, nonfiction, q-and-a format, science, humor, cartoons

Here's a question for you:

Why don't compasses point toward the nearest hospital because of the magnetic fields created by MRI machines?

This question heads Chapter 32 of what if 2: additional serious scientific answers to absurd hypothetical questions by Randall Munroe (Mr. Munroe doesn't capitalize his name on the book cover, but he does in the colophon). This book is the second in a series of, so far, unknown length. In spite of the book's subtitle, this question is not at all absurd, as we find in the first sentence of the answer, "They do, and it can be a problem!", which is followed by this cartoon:

The chapter fills four pages, answering the question in both qualitative and quantitative ways. For example, the author discusses how far from an MRI machine certain sensitive objects must be to be safe from harm or to operate correctly. In the case of a magnetic compass, explorers making their way to the magnetic north pole will be OK as long as they stay at least 10 meters from any MRI machines. Any nearer, and the compass will be offset, leading the explorer toward the machine, like a moth drawn to a candle. The magnetic stripe on a credit card is more robust, and needs to get within 3 meters to be demagnetized (at least partially). If you have a pacemaker, you need to stay 5 meters away, which makes a CT scan the only safe way to be scanned. Later on, the risk of bringing a helicopter to a hospital helipad is discussed, if the helipad has a MRI machine sitting nearby (a recent delivery, perhaps). Potential chaos!

My side note: Most MRI machines have a field strength of 1.5 Teslas (a magnetic unit so-named long before Mr. Musk was born. The magnetic field of the Earth is around 1/10,000 Tesla). It is these that are referred to in this article. A few special purpose MRI scanners have fields as strong as 12 Teslas. By the inverse square law, the square root of 12/1.5 is about 2.8, so our explorer has to stay 28 meters away from one of those, and folks with pacemakers are in danger closer than 14 m.

In toto, the book contains 64 chapters, plus 8 bonuses: 5 "Short Answer" sections with one-line (or one-cartoon) answers, and 3 "Weird & Worrying" sections, containing unanswered questions of various levels of oddity.

Mr. Munroe gathers questions and purveys the answers in one section of his cartoon website, xkcd.com. Another section contains an archive of cartoons, which he issues 3x/wk. Here is an example:

This is item #1741. Getting there, you can navigate anywhere else you want to on the site. I can't begin to describe the incredible variety of subjects he tackles. Publishing his cartoons online gives him great freedom. He isn't restricted to the rules required for cartoons published in newspapers, etc. Some of the cartoons are in the familiar four-panel form, and one that I saw, which has a thermal timeline of the past 18,000 years, with about 2,000 years per page click, is one of the longest web pages I've encountered.

In the book, subjects range from "Earth Eye" (What an eye the size of the Earth could see – it's a lot!) to "Snowball" (How big would a snowball get rolling down the side of Mt. Everest? – not as big as you think) and to "Niagara Straw" (could the flow of Niagara Falls be pushed through a soda straw? – the water velocity would exceed 1/4 the speed of light; the strength of soda straws isn't factored in). Of course, the subjects in what if? 2 are suggested by correspondents, so Mr. Munroe isn't confined to his own imagination. The exercise of imagination needed to provide the answers? That's a whole 'nother kettle of fireworks! And it makes for a delightful book. (I reviewed the prior what if? book and also how to by the same author several years ago.)